diff --git a/browser_patches/firefox-stable/BUILD_NUMBER b/browser_patches/firefox-stable/BUILD_NUMBER
index 3f78fcb7c2..ebdca68403 100644
--- a/browser_patches/firefox-stable/BUILD_NUMBER
+++ b/browser_patches/firefox-stable/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1250
-Changed: yurys@chromium.org Wed 12 May 2021 08:53:32 AM PDT
+1251
+Changed: lushnikov@chromium.org Wed 12 May 2021 04:13:18 PM PDT
diff --git a/browser_patches/firefox-stable/juggler/NetworkObserver.js b/browser_patches/firefox-stable/juggler/NetworkObserver.js
index b97ddc9074..12a3bf355f 100644
--- a/browser_patches/firefox-stable/juggler/NetworkObserver.js
+++ b/browser_patches/firefox-stable/juggler/NetworkObserver.js
@@ -740,11 +740,17 @@ function getSecurityDetails(httpChannel) {
 function readRequestPostData(httpChannel) {
   if (!(httpChannel instanceof Ci.nsIUploadChannel))
     return undefined;
-  const iStream = httpChannel.uploadStream;
+  let iStream = httpChannel.uploadStream;
   if (!iStream)
     return undefined;
   const isSeekableStream = iStream instanceof Ci.nsISeekableStream;
 
+  // For some reason, we cannot rewind back big streams,
+  // so instead we should clone them.
+  const isCloneable = iStream instanceof Ci.nsICloneableInputStream;
+  if (isCloneable)
+    iStream = iStream.clone();
+
   let prevOffset;
   if (isSeekableStream) {
     prevOffset = iStream.tell();
@@ -767,7 +773,7 @@ function readRequestPostData(httpChannel) {
   // Seek locks the file, so seek to the beginning only if necko hasn't
   // read it yet, since necko doesn't seek to 0 before reading (at lest
   // not till 459384 is fixed).
-  if (isSeekableStream && prevOffset == 0)
+  if (isSeekableStream && prevOffset == 0 && !isCloneable)
     iStream.seek(Ci.nsISeekableStream.NS_SEEK_SET, 0);
   return result;
 }
diff --git a/browser_patches/firefox/BUILD_NUMBER b/browser_patches/firefox/BUILD_NUMBER
index 151a0d91ec..83f8b0fc0a 100644
--- a/browser_patches/firefox/BUILD_NUMBER
+++ b/browser_patches/firefox/BUILD_NUMBER
@@ -1,2 +1,2 @@
-1260
-Changed: yurys@chromium.org Wed 12 May 2021 08:49:17 AM PDT
+1261
+Changed: lushnikov@chromium.org Wed 12 May 2021 04:12:52 PM PDT
diff --git a/browser_patches/firefox/juggler/NetworkObserver.js b/browser_patches/firefox/juggler/NetworkObserver.js
index b97ddc9074..12a3bf355f 100644
--- a/browser_patches/firefox/juggler/NetworkObserver.js
+++ b/browser_patches/firefox/juggler/NetworkObserver.js
@@ -740,11 +740,17 @@ function getSecurityDetails(httpChannel) {
 function readRequestPostData(httpChannel) {
   if (!(httpChannel instanceof Ci.nsIUploadChannel))
     return undefined;
-  const iStream = httpChannel.uploadStream;
+  let iStream = httpChannel.uploadStream;
   if (!iStream)
     return undefined;
   const isSeekableStream = iStream instanceof Ci.nsISeekableStream;
 
+  // For some reason, we cannot rewind back big streams,
+  // so instead we should clone them.
+  const isCloneable = iStream instanceof Ci.nsICloneableInputStream;
+  if (isCloneable)
+    iStream = iStream.clone();
+
   let prevOffset;
   if (isSeekableStream) {
     prevOffset = iStream.tell();
@@ -767,7 +773,7 @@ function readRequestPostData(httpChannel) {
   // Seek locks the file, so seek to the beginning only if necko hasn't
   // read it yet, since necko doesn't seek to 0 before reading (at lest
   // not till 459384 is fixed).
-  if (isSeekableStream && prevOffset == 0)
+  if (isSeekableStream && prevOffset == 0 && !isCloneable)
     iStream.seek(Ci.nsISeekableStream.NS_SEEK_SET, 0);
   return result;
 }